Coherent & Non-coherent Introduction

In WDM transmission network, there is Non-coherent and Coherent optical communication. Non-coherent means optical transmission system does not require coherent local oscillator light; Coherent means optical transmission system that uses local oscillator light for coherent detection. This article is sourced from Huawei site, which introduces Non-coherent and Coherent specially.

Overview: Technical Comparison

Item	Non-coherent	Coherent
Definition	Optical transmission system that does not require coherent local oscillator light.	Optical transmission system that uses local oscillator light for coherent detection.
Modulation and demodulation	Transmit end: intensity modulation Receive end: direct detection	Transmit end: external modulation Receive end: coherent detection of local oscillator light
Modulation format	Amplitude modulation (RZ/NRZ/ODB) Differential phase modulation (DQPSK)	Phase modulation (BPSK/QPSK) Quadrature amplitude modulation (QAM)
System structure	Simple Easy to implement and integrate	Complex High technical requirements
Spectral efficiency	Low The frequency and phase information of the optical carrier cannot be used. The single-channel bandwidth is limited.	High Amplitude, frequency, and phase information carried in the optical signal can be detected. The single-channel bandwidth is high.
Dispersion tolerance	Low DCMs need to be configured for dispersion compensation.	High The digital signal processing (DSP) technology is used to offset the fiber dispersion, thereby realizing DCM-free dispersion compensation for long-haul transmission.
ROADM architecture	Complex In the receive direction, the demultiplexer board needs to be used to filter out the corresponding wavelength signals.	Simple Coherent boards can receive specific wavelengths from the multiplexed signals. No demultiplexer board is required.
Application scenario	2.5G and 10G line transmission Early 40G line transmission Multi-subwavelength 100G metro transmission	100G line transmission Super 100G line transmission

Principles of Non-coherent Optical Communication

At the transmit end, service signals are used to adjust the strength (amplitude) of optical carriers. At the receive end, envelope detection is performed on optical carriers to recover the service signals.

Principles of Coherent Optical Communication

In the most commonly used 100G coherent transmission, the transmit end uses high-order modulation such as ePDM-QPSK, and the receive end uses coherent reception technologies. The transmission and reception process is as follows:

1. The polarization beam splitter (PBS) splits the laser light into two perpendicular directions: x-pol and y-pol.
2. QPSK modulation is performed for the optical signals on the x-pol and y-pol. After the serial-parallel conversion, the 112 Gbit/s code stream is divided into four 28 Gbit/s signals.
3. The PBC combines the modulated signals on the x-pol and y-pol onto the same fiber.
4. At the receive end, the received signals are divided into the x-pol and y-pol.
5. The coherent receiver converts the optical signals on the x-pol and y-pol into current/voltage signals.
6. The high-precision analog to digital converter (ADC) converts current/voltage signals into digital bit streams in the format of 0101….
7. The high-speed digital signal processing (DSP) eliminates interference factors, such as dispersion, noise, and nonlinear effects, and recovers the 100G signals transmitted from the transmit end.

Coherent optical communication involves the following key technologies:

– Polarization multiplexing and higher-order modulation: The orthogonal polarization characteristics and phase information of light are used to divide the original signal into two signals for multiple times, which greatly reduces the electrical-layer processing rate required.

– Coherent receiver technology: A local oscillator laser that has the same frequency as the received signal is used to implement interference between the laser signal and the received signal to restore the amplitude, phase, and polarization state information from the received signal.

– DSP technology: The DSP technology is used to resolve signal distortion and latency problems caused by dispersion at the electrical signal layer. It compensates for PMD and CD, greatly improving the PMD and CD tolerance.

– High-performance FEC algorithm: FEC is used to improve the OSNR tolerance of the system. Different FEC types and overhead ratios can be designed for different rates, modulation formats, and transmission performance requirements.

Hybrid Transmission of Coherent and Non-coherent Wavelengths

Coherent and non-coherent signals can be transmitted together under certain conditions. Therefore, professional design is required based on actual engineering conditions. The basic requirements for hybrid transmission are as follows:

– A guard band must be configured between coherent signals and 10G signals or between coherent signals and 40G ODB non-coherent signals to prevent system performance deterioration caused by mutual interference.

– No guard band is required between coherent wavelengths and 40G DQPSK non-coherent signals.

– During hybrid transmission, long wavelengths are preferentially used for coherent signals, short wavelengths are preferentially used for 10G signals, and 40G DQPSK signals are used in the guard band.

– Coherent signals and 10G non-coherent signals cannot be used together on an optical-layer ASON network.